Radio recombination lines (RRLs) in ultra-compact H II Regions have intrinsic widths (FWHM) of about 20 to 50 km/s. However, if there are significant bulk motions of the ionized gas, the lines may be 80 km/s wide (e.g., G5.89-0.39). Electron pressure broadening can be significant at frequencies below about 8 GHz, and the line wings of the Voigt profile can extend to 200 km/s or more. Thus, a total bandwidth of over 300 km/s is necessary to provide minimal spectral baseline, to determine the line profile, and to measure the continuum level. The line-to-continuum ratio is a critical parameter in any model of RRL emission and it is important to measure the continuum under the same conditions as the line.
Every H line is accompanied by the corresponding He, C and X lines
which are spaced at 121 km/s and 150 km/s from the H line (due to the
differences in their atomic weights). The He line is typically 10%
the intensity of the H line. The C line, is weaker still and much
more narrow (
km/s FWHM). All of the lines provide important
information about the physical conditions of the ionized gas and the
gas excitation and dynamics. The He line yields the He abundance
which is an important diagnostic of the chemical evolution of the
Galaxy. The C line is a new way to probe the excitation of
photodissociation regions around the H II regions of high and
intermediate mass stars. It is not possible to cover all these lines
in one band with the present correlator. Ideally one would like to
cover a velocity range of 400-500 km/s. At 
7mm this requires a
spectral line band width of 
MHz. For sensitivity one would
like to obtain the RCP and LCP data simultaneously. To resolve the
narrow C line a channel spacing of 
1 km/s is needed. Thus, more than
500 channels are needed over an approximately 70 MHz band with two
polarizations.
Very broad (
km/s) and in some cases quite asymmetric
RRLs have been observed from ultra-compact H II regions. These
observations, which must be made at the highest VLA frequencies where
the H II regions are optically thin, have been limited in several
respects. First, in order to measure the continuum level, very large
bandwidths are required. The current (recirculating) correlator
severely limits the number of available channels when large bandwidths
are used. Additionally, high spectral resolution is required to
resolve lines which may be blended. Secondly, as discussed above, a
large portion of the continuum emission from ultra-compact H II regions
is very weak and extended, thus producing very weak RRL
emission. Increased continuum sensitivity and increased surface
brightness sensitivity would allow us to determine the kinematics of
the ionized gas over the entire extent of the H II regions.